Many communications carriers, companies, government agencies and corporations that transmit data from one location to another have an interest in providing or having provided for them uninterrupted transmission of data between two distant locations via optical fiber. Currently, to avoid an interruption of service caused by a break in an optical fiber, separate and distinct lightwave paths are utilized. For example, at the transmitting end, a fiber line is split into two, and each of the two fiber links transverse physically diverse paths to a remote destination. At the receiving end, the optical signal in one of the two fibers is selected for use. If, for any reason, there is an interruption in the received signal, a receiver at the receiving end is switched to receive the optical signal from the other or standby line to avoid a partial or complete loss of information for a long period of time. Normally, in standard lightwave systems, terminals, regenerators and other equipment are IXN protected. Thus, a major cause of signal interruption for long duration would be a break in the fiber cable.
Diverse routing can be achieved by one of the following two methods: (A) The use of two physically separate optical fibers to transmit the same signal simultaneously along both paths. This system requires the use of duplicate transmitters, receivers, multiplexers, and the like--one of each for each fiber path; or, (B) By coupling the optical signal from the transmitter into a fused-fiber 3 dB splitter and then connecting the two outputs of the 3 dB splitter to two separate optical fibers. At the receiver site, a simple 2.times.2 electromechanical optical switch (or even a lithium niobate switch) can be used to select the optical fiber from which the signal is to be utilized.
Obviously, the use of a single 3 dB splitter is more economical than the use of the various components for two separate and complete transmitting and receiving systems. However, a main disadvantage of the use of a 3 dB splitter is that it introduces a loss which is excessive for the loss budget provided by most optical systems. Thus, as diverse routing is normally added to existing lightwave systems, the use of 3 dB splitters for diverse routing can create serious signal level related problems.